Karanewsky et al. in U.S. Pat. No. 4,452,790 disclose angiotensin converting enzyme inhibitors of the formula ##STR2## wherein R.sub.1 is alkyl, substituted alkylene, or ##STR3##
Karanewsky et al. in U.S. Pat. No. 4,555,506 disclose angiotensin converting enzyme inhibitors of formula ##STR4## wherein R.sub.1 is hydrogen, alkyl, substituted alkyl, aryl, aralkyl, or heteroalkylene.